99 Tahoe: No 4WD (Page 1/1) RWDPLZ DEC 11, 10:01 PM Trying to figure this one out now. The 4WD switch on the dash has 4 positions: 2HI, Auto 4WD, 4HI, and 4LO 2HI works fine Auto 4WD: the actuator on the front axle does extend, and the light for this switch stays on 4HI: The light on the switch blinks a few times, and the front axle actuator extends, but after about 4 seconds, the actuator retracts, and the switch goes back into 2HI 4LO: Put the truck into neutral, and press 4LO button. The light on the 4LO button will blink for about 30 seconds before simply going back into 2HI The actuator on the front axle does work fine, I pulled it out of the axle and let it hang under the truck, checking what it did each time I pressed a different button. Tried another actuator to be sure, and it does exactly the same as the original actuator. Monkeyman DEC 12, 03:28 AM It should work similar to the 4wd systems in my previous '96 Jimmy and my current '04 Blazer. The electrical plug going to the 4wd module (mine is under the right side kick panel in front of the passenger seat) sometimes has wires break. If you can find that module (I'd start by looking at the same place it is in S10 based trucks), gently pull on the wires in the electrical plug. You might find one (or more) of them is broken/loose. Sometime, tinning the end of the wire and reinserting it is a good temporary fix. RWDPLZ DEC 13, 12:51 PM I figured it out, the main problem was the transfer case encoder motor, which is what shifts the transfer case between the different 4WD modes (Auto 4WD, 4HI, and 4LO), was stuck in the Auto 4WD, which only provides traction to the front wheels if the rears are slipping. Apparently if it fails this way, there is no trouble code stored or 'Service 4WD' light, which made figuring it out a PITA. Had to remove the front driveshaft to get the motor off, which lead to the discovery of another problem: the u-joint on the front of the shaft had been replaced once before, BUT the person who replaced it didn't put the snap rings on the inside of the bearing caps on the ones where the straps are, so THAT would have been a huge mess once the shaft actually came under load. Had to buy a u-joint just to get the snap rings. Once that was done, replaced the 2 quarts of gear oil in the front axle, 3 quarts and a new differential cover in the rear, and replaced the 2 quarts of transfer case fluid. The fluid in the transfer case was red, meaning it was original, or someone replaced it before with the wrong stuff, since the manual transfer cases do call for Dexron III, which is red. The current stuff GM recommends, Auto Trak II, is bright blue.